1. Field of the Invention
The present invention relates generally to a flow measuring device, and more particularly to a flow measuring device in the form of a "U" shaped conduit mounted in beamlike, cantilevered, fashion and arranged to determine the density of a fluid material in the conduit, the mass flow rate therethrough, and accordingly other dependent flow parameters.
2. Description of the Prior Art
Heretofore, flow meters of the general type with which the present invention is concerned have been known as gyroscopic mass flow meters, or Coriolis force mass flow meters. In essence, the function of both types of flow meters is based upon the same principal. Viewed in a simplified manner, Coriolis forces involve the radial movement of mass from a first point on a rotating body to a second point. As a result of such movement, the peripheral velocity of the mass changes, i.e., the mass is accelerated. The acceleration of the mass generates a force in the plane of rotation and perpendicular to the instantaneous radial movement. Such forces are responsible for precession in gyroscopes.
Several approaches have been taken in utilizing Coriolis forces to measure mass flow. For instance, the early Roth U.S. Letters Pat. Nos. 2,865,201 and 3,312,512 disclose gyroscopic flow meters employing a full loop which is continuously rotated (DC type) or oscillated (AC type).
Another flow meter utilizing substantially the same forces but avoiding reversal of flow by utilizing a less than 180.degree. "loop" is described in Sipin U.S. Letters Pat. No. 3,485,098. In both instances, the devices are of the so called AC type, i.e., the conduit is oscillated around an axis and fluid flowing through the conduit flows first away from the center of rotation and then towards the center of rotation thus generating Coriolis forces as a function of the fluid mass flow rate through the loop.
Since there is but one means of generating Coriolis forces, all of the prior art devices of the gyroscopic and Coriolis force configurations generate the same force, but specify various means for measuring such forces. Thus, though the concept is simple and straightforward, practical results in the way of accurate flow measurement have proven elusive.
For instance, the Roth flow meters utilize transducers or gyroscopic coupling as readout means. The gyroscopic coupling is described in Roth as being complex, and transducers are defined as requiring highly flexible conduits, such as bellows. The latter mentioned Roth patent is primarily concerned with the arrangement of such flexible bellows.
Another classical approach for measuring the force proportional to mass flow involve first driving or oscillating a conduit structure through a rotational movement around an axis, and then measuring the additional energy required to drive such conduit as fluid is flowed through the conduit. Unfortunately, the Coriolis forces are quite small compared to the driving forces and, accordingly, it is quite difficult to accurately measure such small forces in the context of the large driving force.
Still another measurement means is described by Sipin at column 7, lines 1 through 23 of U.S. Letters Pat. No. 3,485,098. In this arrangement velocity sensors independent of the driving means are mounted to measure the velocity of the conduit as a result of the distortion of the conduit caused by Coriolis forces. While there may be worthwhile information obtained by such measurements, velocity sensors require measurement of a minute differential velocity superimposed upon the very large pipe oscillation velocities. Thus an entirely accurate determinate of the gyroscopic force must deal with velocity measurements under limited and specialized conditions as discussed below. Mathematical analysis confirms that velocity measurements provide at best marginal results.
If the Coriolis force is not to produce movements of great amplitude, clearly, as a basic precept of physics, a reactive force, or forces, must oppose the Coriolis force. Put simply, the Coriolis force, particularly in the flow meter arrangements permitting distortion of the conduit (a qualification which will be explained below), is opposed by, stated simply, the spring resistance of the conduit itself as it distorts, plus velocity forces resulting from movement of the conduit, i.e., air drag, etc. - usually a most small component - and an inertial component resulting from the acceleration of the mass of the conduit. It is a complex endeavor to concurrently measure and sum all three of these opposing forces. Accordingly, it is understandable that Sipin measures but one of the forces, i.e., velocity, forces. Given the rather involved and marginally accurate conventional mass flow measuring devices utilizing, for instance, independent densities and flow velocities sensors, it is understandable that measurement of a single opposing force such as velocity by Sipin would produce useful though compromised information. If only velocity related reactive forces are to be measured, the other normally more substantially reactive forces should be minimized. This is not the case in the apparatus illustrated by Sipin. No discussion of this critical consideration is to be found.
Another approach to the problem of measuring the small Coriolis forces is described in my U.S. Letters Patent Application Ser. No. 591,907, for "METHOD AND APPARATUS FOR MASS FLOW MEASUREMENT", filed June 30, 1975 now U.S. Pat. No. 4,109,529. In an embodiment of my prior approach, rather than attempting to measure the opposing forces to the Coriolis forces, all of which are dependent upon displacement of the conduit, I describe an arrangement in which a mechanical nulling force, i.e., an opposing force which precludes displacement, is produced. Accordingly, any infinitesimal incremental displacement of the conduit is sensed and opposing force generated. By measuring the opposing force, which replaces the inherent opposing forces described above, an accurate measurement of the mass flow may be made, though at the complication of avoiding spurious measurements of forces resulting from driving the conduit. My application described two independent means for avoiding such complicating forces, i.e., balancing the forces on opposite sides of a beam to cancel the forces and measuring the Coriolis force at maximum angular velocity when driving acceleration forces and bellows spring forces are zero. The balancing approach in conjunction with nulling required relatively slow operation to accomodate the response time of the mechanical beam.
In summary, numerous attempts have heretofore been made to measure mass flow as a function of the Coriolis forces generated by mass flow through an oscillating conduit. However, accurate measurements have been possible only when the conduit displacement is nulled while balancing theacceleration forces due to driving the conduit, and only approximate measurements made when the conduit is allowed to distort against inherent restoration forces such as spring resistance in the conduit, velocity drag factors and inertia while making such measurements.